Retroreflective materials are characterized by the ability to redirect light incident on the material back toward the originating light source. This property has led to the widespread use of retroreflective sheeting for a variety of traffic and personal safety uses. Retroreflective sheeting is commonly employed in a variety of articles, for example, road signs, barricades, license plates, pavement markers and marking tape, as well as retroreflective tapes for vehicles and clothing.
Two known types of retroreflective sheeting are microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes referred to as “beaded” sheeting, employs a multitude of microspheres typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes or vapor coats, etc.) to retroreflect incident light. Cube corner retroreflective sheeting typically comprises a thin transparent layer having a substantially planar front surface and a rear structured surface comprising a plurality of geometric structures, some or all of which include three reflective faces configured as a cube corner element.
Cube corner retroreflective sheeting is commonly produced by first manufacturing a master mold that has a structured surface, such structured surface corresponding either to the desired cube corner element geometry in the finished sheeting or to a negative (inverted) copy thereof, depending upon whether the finished sheeting is to have cube corner pyramids or cube corner cavities (or both). Known methods for manufacturing the master mold include pin-bundling techniques, direct machining techniques, and techniques that employ laminae.
In pin bundling techniques, a plurality of pins, each having a geometric shape such as a cube corner element on one end, are assembled together to form a master mold. U.S. Pat. No. 1,591,572 (Stimson) and U.S. Pat. No. 3,926,402 (Heenan) provide illustrative examples.
In direct machining techniques, a series of grooves are formed in the surface of a planar substrate (e.g. metal plate) to form a master mold comprising truncated cube corner elements. In one well known technique, three sets of parallel grooves intersect each other at 60 degree included angles to form an array of cube corner elements, each having an equilateral base triangle (see U.S. Pat. No. 3,712,706 (Stamm)). In another technique, two sets of grooves intersect each other at an angle greater than 60 degrees and a third set of grooves intersects each of the other two sets at an angle less than 60 degrees to form an array of canted cube corner element matched pairs (see U.S. Pat. No. 4,588,258 (Hoopman)). In direct machining, a large number of individual faces are typically formed along the same groove formed by continuous motion of a cutting tool. Thus, such individual faces maintain their alignment throughout the mold fabrication procedure. For this reason, direct machining techniques offer the ability to accurately machine very small cube corner elements. A drawback to direct machining techniques, however, has been reduced design flexibility in the types of cube corner geometries that can be produced, which in turn affects the total light return.
In techniques that employ laminae, a plurality of thin sheets (i.e. plates) referred to as laminae having geometric shapes formed on one longitudinal edge are assembled to form a master mold. Lamina techniques are generally less labor intensive than pin bundling techniques because fewer parts are separately machined. For example, one lamina typically comprises about 400-1000 individual cube corner elements in comparison to each pin comprising a single cube corner element. Illustrative examples of lamina techniques can be found in EP 0 844 056 A1 (Mimura); U.S. Pat. No. 6,015,214 (Heenan); U.S. Pat. No. 5,981,032 (Smith); U.S. Pat. No. 6,159,407 (Krinke) and U.S. Pat. No. 6,257,860 (Luttrell).
The base edges of adjacent cube corner elements of truncated cube corner arrays are typically coplanar. Other cube corner element structures, described as “full cubes” or “preferred geometry (PG) cube corner elements” typically comprise at least two non-dihedral edges that are not coplanar. Such structures typically exhibit a higher total light return in comparison to truncated cube corner elements. Certain PG cube corner elements may be fabricated via direct machining of a sequence of substrates, as described in WO 00/60385. However, it is difficult to maintain geometric accuracy with this multi-step fabrication process. Design constraints may also be evident in the resulting PG cube corner elements and/or arrangement of elements. By contrast, pin bundling and techniques that employ laminae allow for the formation of a variety of shapes and arrangements of PG cube corner elements. Unlike pin bundling, however, techniques that employ laminae also advantageously provide the ability to form relatively smaller PG cube corner elements.
After manufacturing a master mold the master mold is typically replicated using any suitable technique such as conventional nickel electroforming to produce a tool of a desired size for forming microstructured sheeting. Multigenerational positive and negative copy tools are thus formed, such tools having substantially the same degree of precise cube formation as the master. Electroforming techniques such as described in U.S. Pat. Nos. 4,478,769 and 5,156,863 (Pricone) as well as U.S. Pat. No. 6,159,407 (Krinke) are known. A plurality of replications are often joined together for example by welding such as described in U.S. Pat. No. 6,322,652 (Paulson). The resulting tooling may then be employed for forming cube corner retroreflective sheeting by processes such as embossing, extruding, or cast-and-curing, as known in the art.
For example, U.S. Pat. Nos. 3,684,348 and 3,811,983 describe retroreflective material and a method of making a composite material wherein a fluid molding material is deposited on a molding surface having cube corner recesses and a preformed body member applied thereto. The molding material is then hardened and bonded to the body member. The molding material may be a molten resin and the solidification thereof accomplished at least in part by cooling, the inherent nature of the molten resin producing bonding to the body member thereof. Alternatively, the molding material may be fluid resin having cross-linkable groups and the solidification thereof may be accomplished at least in part by cross-linking of the resin. The molding material may also be a partially polymerized resin formulation and wherein the solidification thereof is accomplished at least in part by polymerization of the resin formulation.
Various retroreflective sheeting comprising truncated cube corner arrays have been commercially successful such as retroreflective sheeting commercially available from 3M Company (“3M”), St. Paul, Minn. under the trade designation “3M Scotchlite Brand Reflective Sheeting 3990 VIP”. Although described in the patent literature, retroreflective sheeting comprising an array of full cubes or PG cube corner elements has not been manufactured commercially or sold. In order to accommodate the commercial success of retroreflective sheeting comprising an array of full cubes or PG cube corner elements, industry would find advantage in improved methods of making retroreflective sheeting comprising such arrays.